I like the idea of players protecting the ecology but if you found that this didn't work perhaps you could use NPCs.

If it were boars that were over killed, then NPC characters would appear who were charged with protecting boars (kinda game wardens?). The more the boars were near extinction the stronger/more numerous these NPCs would become, making it very difficult to wipe out species entirely.

Likewise if it were goblins, then as a player killed more goblins then they would be attacked by goblin assassins. The more goblins they killed the more powerful the assassins.

Likewise if it were goblins, then as a player killed more goblins then they would be attacked by goblin assassins. The more goblins they killed the more powerful the assassins.

The danger is that that could provide some players with an incentive to keep certain populations down, to provide themselves with a regular supply of challenging opponents - and as those players grew in strength they would need stronger challenges, encouraging them to push the NPC population further and further down.

For others, the tough guardians would just provide an added challenge - more incentive to wipe out NPC populations.

As I said earlier in the thread, if it's possible for players to destroy a species, then they will do so. Increasing the difficulty is just delaying the inevitable, and if anything will actually make it a more tempting long-term goal for the players who have already done everything else.

I agree completely. I think it would only make an ecology more robust, it wouldn't solve the underlying problems of players purposely manipulating/vandalising the ecology.

But if the aim is to use the dynamics of the ecology to create story lines and new game play I think perhaps it does achieve that. Or something more advanced along the same lines.

Perhaps an extension of the idea would be that if a guild/clan/city killed a lot of one mob then the defending NPCs would target the entire group. In this way strong players might have to consider what they are killing and how it affects their guild etc. as a whole. I realise it doesn't solve the problem of players destroying the ecology, but it does perhaps create some interesting game play.

The form of defense wouldn't have to be NPCs either. It could be penalties like certain areas becoming inaccessible, certain skills/items becoming unlearnable (kill too many thieves the thief who teaches pickpocketting won't help you).

I realise that no matter the penalties it is only going to discourage and make ecological damage more difficult. I agee that ultimately if it is possible it will happen, and the challenge might act as an incentive even. The ecology would have to be designed in a way that species destruction was impossible or species were replaced/respawned etc.

However I really like the idea of ecological impact have consequences for the player, even if the aim is to create a new element of gameplay rather than to allow a sustainable ecology.

Old discussion - still, some of you might find this material interesting:
http://home.comcast.net/~sharov/popechome/
quantitative population ecology, the science of how many boars and goblins form a tabl equilibrium. One interesting part (for MUD-programmers) are the Petri-nets, described here:
http://home.comcast.net/~sharov/PopEcol/lec1/petrinet.html
this is n iterative way of calculating populations, thus simpler and better suited for computer games than a system of differential equations (though you'd need to formulate and solve those equations once to find stable starting conditions).
In a computer-game environment, you could replace the transitions with scripts, to allow some flexibility in predator-prey relations (if deer density<10, dragons eat PCs).

If you want a stable ecology, you'd need some stabilising factors outside those mechanisms. those could be apex-predators who switch their behaviour at certain tipping points, game wardens or whatever. To address vandalism, "game wardens" would be sufficient in my opinion - they just have to keep the respective player too buy to grind boars, If the player enjoys his new tough opponents, the better - we want players to have fun!

A fully simulated ecology could be a sort of plot device, to ideas to that effect:
druids/elves/werewolves draw some power from being in a healthy environment (species distribution close to stable equilibrium), and thus have an active interest in hunting down excessive hunters - pk galore!
Mad scientists/mages introduce new species into an unstable ecology, to see whose pets drive out the others. Players can join the fun as hunters, mad scientist, or simply as people trying to live their lives between the armored sheep and the radioactive squirrels.
Building successfull species and having them compete could be a game in itself, and one that cashes in on one strength that MUDs have - it's theoretically easy to have lots of player generated content, the players need only a solid grasp of one language (vs. graphical design skills).

For inspiration, think princess Mononoke or sunken gardens (a short story by bruce sterling, in which different factions enter a contest in designing ecosystems to terraform mars).

p.s. reading sunken gardens actually got me into digging up my old ecology scripts, and remembering this discussion, and daydreaming about a game where ecosystem design is a fundamental aspect.